ST. LOUIS – The Plains states where the production of corn and soybeans is key are being hit harder by excessive drought conditions in the wake of the hottest month on record in the continental U.S., contributing to a surge in global food prices.

The weekly U.S. Drought Monitor map released Thursday showed that the amount of the contiguous U.S. mired in drought conditions dropped a little more than 1 percentage point, to 78.14 percent as of Tuesday. But the expanse still gripped by extreme or exceptional drought – the two worst classifications – rose to 24.14 percent, up nearly 2 percentage points from the previous week.

That’s because key farm states didn’t get as much benefit from rains as elsewhere on the heels of temperatures in July that federal scientists said were so high they broke a record set during the Dust Bowl of the 1930s.

Growers in Iowa – the nation’s biggest corn and soybean producer – saw their conditions further deteriorate, with the amount of that state in extreme or exceptional drought more than doubling from 30.74 percent last week to 69.14 percent now.

In neighboring Nebraska, the expanse of land considered in the two worst drought categories rose to 91.2 percent, up 8 percentage points.

The amount of Kansas in exceptional drought also more than doubled, up to 38.58 percent from 17.45 percent, while extreme or exceptional drought in Illinois spiked roughly 10 percentage points, to 81.18 percent.

“Same song, tenth verse last week as much of the Plains saw the pattern of excessive heat and dryness persist, leading to more expansion across Nebraska, Kansas, Oklahoma and parts of Texas,” Mark Svoboda, a climatologist with the National Drought Mitigation Center, wrote in Thursday’s report.

Those dry conditions have factored into a sharp rise in global food prices after three months of decline, the U.N.’s Rome-based Food and Agriculture Organization said in its monthly price report Thursday.

Severe drought punishing the U.S.’s midsection has sent corn prices soaring by almost 23 percent, and expectations of worsened crop prospects in Russia because of dry weather sent world wheat prices up 19 percent, according to the FAO, which keeps close tabs on volatile global prices. Spikes in the prices of staple foods have led to riots in some countries in recent years.

The U.S. leads the world in exporting corn, soybeans and wheat, and the surging prices are expected to be felt across the international marketplace, hurting poor food-importing countries, said a study by British charity Oxfam issued on the eve of the U.N. report.

The FAO said its overall food price index climbed 6 percentage points in July, although it was well below the peak reached in February 2011. The FAO’s index, considered a global benchmark used to track market volatility and price trends, measures the monthly price changes for a basket of food items including cereals, oils and fats, meat, dairy products and sugar.

Corn, wheat, soybean and sugar prices rose. But on a brighter note, the July report said the price of rice and dairy products remained constant and the agency’s meat price index fell.

Nonetheless, Oxfam cautioned that “millions of the world’s poorest will face devastation” because of the rising prices.

“This is not some gentle monthly wake-up call – it’s the same global alarm that’s been screaming at us since 2008,” Oxfam spokesman Colin Roche said after the latest figures were released. “These new figures prove that the world’s food system cannot cope on crumbling foundations. The combination of rising prices and expected low reserves means the world is facing a double danger.”

After months of drought, the central Illinois creeks and ponds that the 300 cows and calves drink from on Tony Frost’s farm in Tallula, Ill., are dry or close to it. Frost has to buy and haul water, about 4,000 gallons a day.

Seth Perlman/Associated Press

After months of drought, the central Illinois creeks and ponds that the 300 cows and calves drink from on Tony Frost’s farm in Tallula, Ill., are dry or close to it. Frost has to buy and haul water, about 4,000 gallons a day.